I would say that it is more than the serum, but "carrying the shield" is just a term for becoming Captain America.hurleyag said:
Someone explain to me why Bucky or Sam carrying the shield make them "Captain America?" I get that the shield is strong, but the serum is what makes Steve "Captain America."
It appears in the MCU Bucky has some super speed, strength, etc. so it wouldn't be a big stretch at all if he fought like him and acted like him physically. Plus we all know the Vibranium arm is coming.
Plus they would get new costumes to sell.